1. Field of the Invention
The present invention relates to an information processing method, information processing apparatus, program and medium of an image forming apparatus such as a color printer.
2. Description of Related Art
An image quality evaluation method of output images of image forming apparatuses such as printers are divided into a psychological evaluation that evaluates the degree perceived by vision, and a physical evaluation that evaluates by objectively measuring values of properties of image structures. As one of the important factors of the image quality, there is gradation of an image.
As a standard for representing the gradation physically, a method of obtaining a “distinguished gray scale” is used which divides the density range of a patch image, in which a pixel value has a fixed area separated apart at fixed intervals, by twice the RMS granularity (it will be called conventional technique 1). Although the algorithm of the conventional technique 1 can calculate the distinguished gray scale of a printer, it cannot evaluate an image such as a gradation image whose color or lightness continuously changes from shadow to highlight. Even if the interval is made small of the input pixel values of the patches used for calculating the distinguished gray scale, accurate gradation evaluation cannot be achieved because of the following reasons. First, colors and lightness can change in accordance with printing positions because of the effect of noise superposed onto the printing regions when the printer carries out printing or of the shading characteristics of the printer. Second, using a patch image as the evaluation image presents a problem in that since the impression, which is given by an image such as a gradation image whose colors and lightness gradually vary in very small regions, differs from the impression perceived by human vision, matching is not good between the evaluation values.
To solve the problem, a method is proposed which carries out the so-called objective evaluation of a gradation image output from an image forming apparatus. For example, Japanese patent application laid-open No. 11-25274/1999 discloses an image evaluation method of such a kind (called conventional technique 2 from now on). The method, using at least one of the optical information items including the lightness information and density information on the image output, calculates the difference between the optical information items on adjacent gray levels, thereby objectively evaluating gradation characteristics of the output apparatus.
However, according to the conventional technique 1 or the conventional technique 2 which is the improved method of the former and is disclosed in the foregoing document, the gradation evaluation is not performed for all the gradations distributed three-dimensionally, but is carried out only along one axis of the lightness or density. In addition, even if the evaluation values can be calculated, their feedback cannot be performed directly.